Calcium Phosphate Mineralization beneath a Polycationic Monolayer at the Air–Water Interface

Abstract

The self-assembly of the amphiphilic block copolymer poly(n-butyl methacrylate)-block-poly[2-(dimethylamino)ethyl methacrylate] at the air-water interface has been investigated at different pH values. Similar to Rehfeldt et al. (J. Phys. Chem. B 2006, 110, 9171), the subphase pH strongly affects the monolayer properties. The formation of calcium phosphate beneath the monolayer can be tuned by the subphase pH and hence the monolayer charge. After 12 h of mineralization at pH 5, the polymer monolayers are still transparent, but transmission electron microscopy (TEM) shows that very thin calcium phosphate fibers form, which aggregate into cotton ball-like features with diameters of 20 to 50 nm. In contrast, after 12 h of mineralization at pH 8, the polymer film is very slightly turbid and TEM shows dense aggregates with sizes between 200 and 700 nm. The formation of calcium phosphate is further confirmed by Raman and energy dispersive X-ray spectroscopy. The calcium phosphate architectures can be assigned to the monolayer charge, which is high at low pH and low at high pH. The study demonstrates that the effects of polycations should not be ignored if attempting to understand the colloid chemistry of biomimetic mineralization. It also shows that basic block copolymers are useful complementary systems to the much more commonly studied acidic block copolymer templates.

Crystal structure and chemical composition of biomimetic calcium phosphate nanofibers

“…The resulting fibers are morphologically very similar to those obtained previously (diameters below ca. 5 nm and lengths in the micrometer range) and to fibers reported in the literature, 27,28 but BF-TEM shows that mineralization from 20 mM solutions leads to more electron beam-resistant fibers and to a much higher number of fibers than mineralization from 2 mM solutions 13,14 ( Fig. 1).…”

“…To simplify the fiber synthesis we have, in contrast to our previous studies, 13,14 not buffered the reaction mixture, but used the same block copolymer and the same salt concentrations (2 mM) as in the previous work. Bright field transmission electron microscopy (BF-TEM) shows that this does not affect the outcome of the mineralization as in all cases fibers with diameters of a few nanometers form.…”

“…, PDI 1.10, the subscripts denote the degrees of polymerization for the corresponding segment) has been described before. 14 …”

“…[11][12][13][14][15][16][17][18][19] Likewise, the effects of (poly)cations on calcium phosphate and calcium carbonate mineralization have only recently begun to be addressed. 13,14,[19][20][21][22][23][24][25][26] According to these studies, there are indeed significant differences between (i) bulk and surface mineralization and between (ii) polyanion and polycationcontrolled calcium phosphate mineralization. Among others, poly(cation) monolayers at the air-water interface template the formation of intriguing fibers with diameters of only a few nanometers.…”

“…Among others, poly(cation) monolayers at the air-water interface template the formation of intriguing fibers with diameters of only a few nanometers. 13,14 As the fibers are many micrometers long and form extended structures, they could be interesting for the construction of hybrid biomaterials or as fillers for mechanical stabilization of composite biomaterials. Interestingly, the fibers resemble examples that have been made via precipitation from bulk solution using negatively charged polymer additives.…”